Cellular structure is a basic principle for the configuration of the radio subsystem of a mobile communication system, and the service coverage of the system can be expanded by addition of base stations.
Typically, in the cellular communication system, a user experiences low Signal To Interference and Noise Ratio (SINR) at the cell boundary far from the serving base station due to the signal loss and neighbor cell interferences. In order to maintain the ongoing service in such a situation, it is required to reduce the amount of transmission data or increase resources such as frequency, time, and transmission power as compared to the near users.
Meanwhile, the cellular communication system supports handover from the serving base station to a neighbor base station to secure user mobility. There are two types of handovers: hard handover in which a connection is broken before a new radio connection is established between the mobile station and the base station and soft handover in which a new radio connection is established before the old connection is broken between the mobile station and the base station.
Typically, the 2nd Generation (2G) and 3rd Generation (3G) Code Division Multiple Access (CDMA) systems adopt the soft handover in uplink direction and the hard handover in downlink direction. However, Orthogonal Frequency Division Multiple Access (OFDMA)-based 4G standards such as Long Term Evolution (LTE) of the 3rd Generation Partnership Project (3GPP) specify the hard handovers in both the uplink and downlink directions. In the CDMA system, a base station can receive the signal transmitted by a mobile station served by another base station, if it knows the channelization code of the mobile station. However, in the OFDMA system which allocates frequency and time resources per mobile station, multiple base stations have to share the resource allocation information for allocating the same frequency and time resources to a single mobile station. That is, the early OFDMA-based 4G system is designed under the assumption of hard handover in consideration of the costly communication between the base stations.
Historically, handover has been considered mainly as an issue related to mobility, however, various researches are conducted recently to improve the communication stability at the cell boundary areas of bad channel conditions. Cooperative network is a concept in which multiple base stations cooperate to support single mobile station based on the shared information. In contrast, distribute network is a network architecture operating without information exchange among base stations.
The cooperative networks can be classified depending on the information level exchanged between the base stations. If multiple base stations share the information destined to the same mobile station, it is possible to implement a cooperative network to transmit the data to the mobile station simultaneously. If multiple base stations share the information on the frequency and time resources and power allocated to a mobile station but not the data destined to the mobile station, it is possible to implement a cooperative network to avoid inter-cell interferences. In the former case, the information amount exchanged between the base station increases, but equivalent channel condition is improved from the viewpoint of the mobile station. In the latter case, it is difficult to expect the improvement of the equivalent channel condition to that extent in the former case, but any improvement with the cost of tiny increase of information exchange between the base stations.
In the meantime, beamforming is a technique to improve the equivalent channel condition of the user by applying beamforming weights to multiple antennas of the base station individually for optimal channel of the user. The beamforming weights have to be adjusted individually in adaptation to the time-varying channel characteristics. If the spatial characteristics of the channels are known, the base station can compute the beamforming weights. In uplink direction, the base station can compute the appropriate beamforming weights of the antennas using the reference signals transmitted by the mobile station for demodulation. In case that the spatial characteristics of the channel are not known, the base station has to receive feedback signal transmitted by the mobile station for acquiring the spatial characteristics of the channel. In the Frequency Division Duplex (FDD) system, the base station has no way to acquire the spatial characteristics of the channel in downlink direction, thereby requiring channel condition report from the mobile station.
The feedback information transmitted by the mobile station for supporting beamforming weight computation of the base station includes Channel Matrix Indicator (CMI) and Precoding Matrix Indicator (PMI). Since the base station is provided with plural transmit antennas, the channel conditions are expressed in the form of vector or matrix. The feedback information is quantized to improve the reliability such that a channel matrix set composed of a predetermined number of elements representing the channel vectors or matrices is defined. The mobile station selects a channel vector or matrix indicative of its channel condition from the channel matrix set and reports the CMI indicating the selected channel vector or matrix to the base station. The PMI is fed back to report the user preference beamforming weights. The mobile station selects an element from a beamforming weight matrix composed of a predetermined number of elements representing the beamforming weight available for the base station and reports the PMI indicating the selected beamforming weight to the base station.
FIG. 1 is a schematic diagram illustrating a transmission beamforming in a conventional distributed network cellular structure.
In FIG. 1, the mobile station 105 is connected to the base station 101, the mobile station 107 is connected to the base station 103, and the base stations 101 and 103 do not share the information related to the beamforming and resource allocation. The base station 101 forms its beam as denoted by reference numeral 111 to transmit a signal to the mobile station 105, and the base station 103 forms its beam as denoted by reference numeral 113 to transmit a signal to mobile station 107. Since the two base stations 101 and 103 do not share information, the beam 111 and 113 are formed independently. In this case, the base station 101 focus the beam 111 to improve the SINR for the mobile station 105, however, the beam 111 may be influenced by the beam 111 formed by the base station 103. That is, the signal transmitted by the base station 103 can act as storing interference to the mobile station 105, whereby the SINR improvement effect of the beam 111 is nullified. This means that the independent beamforming of the base stations do not guarantee the SINR improvement for the mobile station.
FIG. 2 is a schematic diagram illustrating a transmission beamforming in a conventional cooperative network cellular structure.
In FIG. 2, the mobile stations 107 and 207 are connected to the base station 103. Meanwhile, the base station 101 schedules the mobile station 105 to form the beam 111. In this situation, if the base station 103 forms a beam for transmitting signal to the mobile station 107, the beam focused to the mobile station 107 is likely to make interference to signal transmitted from the base station 101 to the mobile station 105 as in FIG. 1. In the cooperative network, however, a cluster controller 201 controls the scheduling operations of the two base stations 101 and 103, such that the two base stations 101 and 103 can perform scheduling and beamforming without interfering with each other. With the cooperation between the base stations 101 and 103, the base station 103 forms the beam 213 focused to the mobile station 207 while the base station 101 forms the beam 111 focused to the mobile station 105 far from the mobile station 207 enough to avoid interference, it is possible to guarantee the improvement of SINRs of the downlink channels of the base stations 105 and 207.
The cluster scheduler 201 can be located near the base station 101 or 103 geographically or at a remote place. The cluster scheduler 201 makes a determination for interference-free scheduling, power allocation, and beamforming and reports the determination result to the base stations under the control of the cluster scheduler 201. Here, the term cluster means a set of the base stations under the control of the scheduler.
FIG. 3 is a sequence diagram illustrating operations of network elements in a distributed network cellular structure of FIG. 1 for channel sensitive scheduling.
Referring to FIG. 3, the mobile station 301 is connected to the base station 303. In order for the base station 303 to perform the channel sensitive scheduling, the mobile station 301 sends channel condition information such as Channel Quality Indicator (CQI) and Precoding Matrix Indicator (PMI) to the base station 303 (305). The base station 303 receives the feedback information such as the CQI and/or PMI from all the mobile stations within the cell, generates resource allocation and beamforming information based on the feedback information, and sends the resource allocation and beamforming information to the mobile station 301 in the form of control information.
FIG. 4 is a sequence diagram illustrating operations of network elements in a cooperative network cellular structure of FIG. 2 for channel sensitive scheduling.
Referring to FIG. 4, the mobile station 301 is connected to the base station 303. The mobile station 301 sends channel condition information such as CQI and PMI to the base station 303 in order for the base station to perform channel sensitive scheduling and beamforming in consideration of the interference with neighbor base station 403 (405). At this time, two types of PMIs can be transmitted: one intended to be used by serving base station 303 and the other intended not to be used by the neighbor base station 403.
The serving base station 303 collects the feedback information transmitted by the mobile stations within the cell and delivers the feedback information to the cluster scheduler 401 (407). The neighbor base station 403 also collects the feedback information transmitted by the mobile stations within its cell and delivers the feedback information to the cluster scheduler 401 (409). The cluster scheduler 401 generates downlink scheduling information including scheduling, resource allocation, and beamforming parameters for the base stations 303 and 403 under its control and sends the downlink scheduling information to the base stations 303 and 403 (411). Upon receipt of the downlink scheduling information, the base station 303 sends the control information generated based on the downlink scheduling information to the mobile station 301 along with the transmission signal (413).
The cooperative network architecture has been introduced to improve downlink SINR of the mobile stations. However, the conventional cooperative network does not help improving the uplink SINR of the mobile station at the cell boundary area. Furthermore, transmitting additional feedback information, i.e. PMI intended not to be used by the neighbor base station, as well as the CQI and PMI intended to be used by the serving base station must be burdensome from the viewpoint of the mobile station located at the cell boundary area. That is, the conventional cooperative network must pay significant cost in uplink direction for improving the SINR in downlink direction.